Presently, in the fabrication of pneumatic tires it is known that a green tread extrudate is "stitched" to the tire body for the formation of a tread thereon during the curing operation. With reference to FIG. 1, it can be seen that a typical green tread extrudate, as presently known, is designated in cross-section by the numeral 10. The extrudate 10 includes a base stock material 12, typically of rubber composition, adapted to be adhered to the tire body upon curing. A cap stock 14, of a different rubber composition, is adhered to the top of the base stock 12 as shown. As is well known in the art, the tire tread is formed in the cap stock 14 in standard fashion.
The specific configuration of the extrudate 10, as well as the nature and configurations of the base stock 12 and cap stock 14 are dictated by the tire specifications on which the extrudate 10 is to be used. Parameters such as tire diameter, width, and tread design all dictate the design criteria for the extrudate 10 and the constituent layers 12,14. Not only is the overall geometry of the extrudate 10 important, but it has been found that the junction 16 between the cap stock 14 and base stock 12 is equally important, if not more so. As shown in FIG. 1, this junction angles upwardly at each end of the extrudate 10 as designated by the numeral 18. These areas, which correspond to the shoulder of the tire, are characterized by a thinning of the cap stock 14 and a thickening of the base stock 12. The relative thicknesses of the layers 12,14 at various points in the extrudate 10, and the degree and length of the angle portions 18 is of great significance in tire fabrication and is dependent upon the specifications accorded the resulting tire. Since a large variety of tire specifications are now known, the capability of effectively, reliably, and economically controlling the extrudate geometry is paramount.
As shown in FIG. 2, the prior art has taught that the extrudate of FIG. 1 may be made utilizing a die assembly designated generally by the numeral 20. Such a die assembly would typically include a preform die 22 having various openings and passages machined therein dependent upon the desired nature of the extrudate to be formed. An opening and passage 24 would be machined in the preform die 22 and would communicate with a suitable conduit for passing the cap stock material therethrough. In like fashion, an opening and passage 26 would be formed in the preform die 22 and communicate with a suitable conduit for passing the base stock material. Side extrusion openings and passages 28,30 are similarly machined to obtain the desired geometry of the resulting extrudate 10. As can be seen, the side extrusion openings 28,30 are machined such as to seek to obtain a desired geometry of the junction between the layers 12,14. Of course, to obtain the approach angle as at 18, the passages and openings 28,30 must be carefully designed and machined.
A final die 32 is secured to the housing 22 by means of screws 34 or other appropriate fasteners. While the final die 32 is only illustratively shown in FIG. 2, it will be understood that the die 32 communicates with the openings and passages 24-30 to form the final extrudate 10. However, the die portion of the final die 32 is configured to be substantially identical to the external geometry of the extrudate 10, to only define such external geometry, and not to define the junction 16 between the base stock 12 and the cap stock 14. Typically, the final die 32 only defines the upper geometry of the cap stock 14. Again, the junction 16 is defined by the machining of the passages and openings 24-30 in the housing 22. Accordingly, to provide green tire tread extrudates for a wide variety of tire specifications, a large number of preform dies 20 have been found to be necessary. As will be readily appreciated, the generation of each of the preform dies 20 is both expensive and time consuming, requiring extensive operations by machinists and other skilled tradesmen.
A number of prior art teachings are also known which teach or suggest techniques and apparatus for use in extruding pneumatic tire tread composites. U.S. Pat. No. 2,569,373 presents a die head for generating a multilayered extrusion for use in forming pneumatic tire treads. However, the structure presented therein fails to keep the base stock and cap stock separated until entering the final die. U.S. Pat. No. 3,941,551 teaches coextruding of layers of material and includes an adjustment feature whereby the profile of the outlet slots of the extruder may be adjusted while the machine is in operation. However, it is believed that such an apparatus is readily given to loss of adjustment during operation. U.S. Pat. No. 3,584,343 teaches a rubber extrusion head having a paired pre-template and finish template, but does not teach the utilization or control of separate base and cap stocks.
Of general interest, applicants are aware of U.S. Pat. Nos. 3,870,453 and 3,877,857. These patents teach an extruder structured to control the gauge, but not the contour, of an extrudate. In somewhat similar fashion, U.S. Pat. No. 4,329,133 presents an extruder head utilizing knives to control relative thicknesses of the extruded material.